I found out my dad is having affairs with other women (he's married) what do I do??

Ok my dad has already been married three times. Idk how he lost his first wife. He lost his second wife (my mom) because he cheated on her with my now stepmom. He cheated because my mom was a jerk which is true, she was. My dad apologized to me for the incident. Now my dad is married to my stepmom and he's cheating again! Using skype to talk to women WTF. My stepmom is so cool and nice. Sure they had their ups and downs but WTF. Why would he cheat on my nice stepmom? I found out he was cheating by looking at his skype chats. Me and my dad get along well. Idk what to do. I cant sleep at night and everyone is gonna think I'm a big mouth if I tell. Why is my life so screwed up like this?????